Because the thinking at the time appears to be that to "ease' renumbering
reduce the costs associated with address distribution functions (and
associated network assessment tasks) and because there were heaps of
addresses, all end-sites would get the same address allocation, and the
uniform amount that was arrived at was a /48 . When asked whether this
referred to _everything_ that may require subnets, the answer was "yes".
When asked whether this encompassed everything from a mobile phone to a
large corporate the answer given was, once more, "yes".

Why /48 rather than /47 or /49? - alignment to nibble boundaries to make
DNS delegation easier.

Why /48 rather than /32 or /40? I really cannot say - I suspect that /48 is
the largest end site number that meets the projected scope as described in
RFC 3177.